The present invention relates to a display control system for controlling a raster scan type display apparatus, and more particularly to a display apparatus in which a plurality of split display regions are scanned in parallel.
In a display apparatus, such as a cathode ray tube, the whole display region or screen is usually scanned by one scanning line raster. On the other hand, there are different types of display apparatus, for example, a liquid crystal display apparatus, in which a display region or screen is divided into two or more split display regions and the plural display regions are scanned in parallel.
In the ordinary display system which scans a plurality of split display regions in parallel, a random access memory is used as a memory for storing bit map display data to be displayed; and, drawing access for storing display data into the display memory and display access for reading out the stored display data are alternately carried out on a time-sharing basis.
This time sharing display system is able to designate addresses at random during the display access operation and, therefore, to alternately read out the display data from two or more memory regions disposed separately from each other and to supply the plurality of display data to the display device.
The display control system which uses a random access memory can realize a wrap-around scrolling by controlling the reading address of the display memory as described in Japanese Laying-open Patent Publication Sho-63-213,888.
In case a random access memory is used as a display memory in a high definition large scale display apparatus, the amount of data per unit time period read out from the display memory becomes large and competition between the display access operation and the drawing access operation of the display memory increases. As a result, the drawing speed becomes slow.
The above problem is solved by using a dual port display memory (hereinafter referred to as DPRAM) which is able to execute these two accesses simultaneously. In accordance with this technique, since the reading of data for display from the serial port of the DPRAM is executed by continuous addresses, the display addresses are required to be arranged so as to alternately read the data of plural display regions, for example, an upper display region and a lower display region, from the display memory using a time-sharing system. This means that the display addresses of the plurality of display regions have to be aligned alternately on the basis of the order of the continuous read addresses for the display memory and each data item of the respective display regions should be stored alternately in the display memory during the drawing process.
When only a partial region of the display is scrolled or shifted by varying the starting address of the read access, as shown in Japanese laying-open Patent Publication Sho 63-213888, this system causes the disadvantage that the display region which is not to be scrolled is also scrolled because one serial address is used for displaying.
Furthermore, the above technique has another disadvantage that the memory map of the display is quite different from that used in the ordinary random access memory and ordinary drawing software can not be used.